Motor vehicles are typically equipped with external lights to indicate their approach to nearby observers. In addition to turn signal lights, and headlights for nighttime operation, most vehicles are provided with at least one brake or stop indicator light. In known vehicle signal light systems, the stop indicator lights have been unsatisfactory because if they are of sufficient luminosity to command attention during daylight driving conditions, they are too dim or indistinct to be readily observed during nighttime use. A need, therefore, exists for an improved vehicle signal light system having means for automatically adjusting the intensity of a stop indicator light for daytime or nighttime driving conditions.